hypocoristic
Concise Oxford English Dictionary © 2008 Oxford University Press:
hypocoristic /ˌhʌɪpə(ʊ)kəˈrɪstɪk/
▶adjective of or denoting a pet name or diminutive form of a name. ▶noun a hypocoristic name or form.
– derivatives
hypocorism noun.
hypocorism noun.
– origin C19: from Gk hupokorisma, from hupokorizesthai ‘play the child’.
'hypocoristic' also found in these Oxford entries:

